1. Field of the Invention
The present invention relates to a displacement sensor which measures a displacement of a measuring object in a non-contact state.
2. Description of the Background Art
In the technical field such as FA (factory automation), many displacement sensors have been used in positional controlling processes for manufacturing devices used for manufacturing products or positional controlling processes for products with respect to the manufacturing devices, or inspection processes for products and the like. These displacement sensors generally use a triangulation surveying system. In this system, light is applied to a measuring object, and the light reflected by the measuring object is detected by a position detection element and the like, and based upon the positional change in the center of gravity of a light-receiving spot on the position detection element that is changed in accordance with the displacement of the measuring object, the amount of displacement is measured.
In this displacement sensor in the triangulation surveying system, it is not possible to maintain the size of the light spot applied on the measuring object at a fine spot size corresponding to a diffraction limit. As shown in FIG. 48, even when coherent light from, for example, a laser 1001 serving as a light source is converged by using a lens 1002, the spot size corresponding to the diffraction limit is obtained only at one point indicated by reference numeral 1006, as indicated by a solid line, and the spot size at almost all the other positions is expanded. Even when the reflected light derived from such expanded irradiation light on the measuring object 1005 is converged on a position detection element 1004 by a lens 1003, the resulting light-receiving spot has an expanded form as indicated by a broken line.
Here, any measuring object has irregularities in reflection intensity due to fine roughness on the surface and color unevenness as long as it does not have a complete mirror surface. Consequently, luminance irregularities occur in the reflected light spot on the measuring object, resulting in a positional change in the center of gravity of the light-receiving spot on the position detection element. In other words, even when the amount of displacement in the measuring object is 0, a positional difference on the surface of the measuring object causes a change in the position of the center of gravity of the light-receiving spot on the position detection element due to irregularities in the reflection intensity, with the result that the displacement measured value as the result of measurements becomes different. This causes a measuring error, and disturbs measuring processes with high precision.
With respect to a displacement sensor that prevents this measuring error, conventionally, a device has been proposed in Japanese Patent Application Laid-Open No. 7-113617. In the technique disclosed in this gazette, different from the triangulation surveying system, by sweeping the lens in the displacement measuring direction, the light-converging position of irradiation luminous fluxes is changed, and based upon the fact that when the light-converging position of the irradiation luminous fluxes is coincident with the position of the measuring object, the size of the light-receiving spot of the reflected light is minimized, the amount of displacement is measured. In accordance with this method, since the position of the center of gravity of the light-receiving spot of the reflected light is not utilized, it is possible to carry out displacement measurements without having any adverse effect from the irregularities in reflection intensity.
However, in the displacement sensor of this type, since the sweeping process is carried out by using the lens attached to a tuning fork, there is a limitation in increasing the sweeping frequency. This is because, the lens, which needs to have a predetermined size so as to reduce the spot diameter that is determined by the diffraction limit in order to maintain a sufficient amount of receiving light, cannot be miniaturized. This causes a reduction in the natural frequency, making it difficult to increase the sweeping frequency. The low sweeping frequency makes longer the time required for measurements for the amount of displacement at one point. In the case of measurements at a plurality of points or in the case when measured values at the same point are averaged to obtain measured values with higher precision, the total measuring time becomes longer in proportion to the number of measuring points or the number of averaging processes.
Further, the displacement sensor of this type is often used for measurements of the shape of an object with high precision, and in such a case, a great number of measuring points are required, and averaging processes for measurements with higher precision are often required. In an attempt to measure the shape of a measuring object with high precision, although it depends on applications, the total measuring time often reaches an order of seconds to minutes, causing a step that limits the number of products. When some of the inspection processes are omitted in order to shorten the total measuring time, defective products might escape from rejection, and when the number of averaging processes of measured values is reduced, there is degradation in the measuring precision.
Moreover, since the natural frequency is determined integrally by the lens and tuning fork, and since the sweeping frequency is virtually determined by the natural frequency, it is not possible to easily alter the detection distance and the displacement measuring range by exchanging lenses. This is because the exchange of lenses requires the alteration in the sweeping frequency, causing changes in the processing circuits, the tuning-fork-driving coils and the like. In an attempt to make changes in the detection distance and displacement measuring range without changing the sweeping frequency, it is necessary to change the optical characteristics of the lens in an equalizing manner by adding a number of lenses in the light path in a separated manner with the lens integral with the tuning fork being maintained as it is, or to redesign the entire optical system so that, after the lens to be subjected to the sweeping process having been altered, the sweeping frequency is maintained at the same value. When a number of lenses are added, the outside shape becomes larger, causing high costs. In the case when the sensor is redesigned and manufactured in response to the detection distance and displacement measuring range corresponding to the changed measuring object and shape of the device, high developing costs and the like are required, also resulting in high costs.
The present invention has been devised to solve the above-mentioned conventional problems, and its objective is to provide a displacement sensor which can carry out displacement measuring processes with high precision without receiving virtually any adverse effects from irregularities in the reflection intensity, and shorten the time required for measurements.
Further, another objective of the present invention is to provide a displacement sensor which has an arrangement capable of exchanging lenses so that the detection distance and the displacement measuring range are easily altered, and is also adapted for various individual applications.
Moreover, still another objective of the present invention is to provide a displacement sensor which is designed to easily select lenses of different kinds at the time of manufacturing processes, and easily manufactured at low costs while being adapted for a combination selected from a wide range of combinations between the detection distance and the displacement measuring range.